1. Field of the invention
The present invention relates to a low-pressure mercury vapor discharge lamp, and more particularly to a low-pressure mercury vapor discharge lamp made of a single piece glass tube bent into a generally U-shaped configuration.
2. Description of the Prior Art
The bent type low-pressure mercury vapor discharge lamp such as a fluorescent lamp, basically, has a tubular discharge envelope bent once or several times, which consists of several straight, parallel longitudinal tube sections and at least one transverse connecting tube section. The transverse connecting tube section connects a pair of the longitudinal tube sections so that the longitudinal tube sections are united into a continuous bent tube together with the transverse connecting tube section.
Opposite ends of the resultant continuous bent tube are aligned in the same direction and sealed. The sealed ends are provided with stems for supporting discharge electrodes, and covered by a base member which permits the low-pressure mercury vapor discharge lamp to be used in a suitable fixture. The discharge electrodes are electrically connected to terminal pins mounted on the base member. This structure provides a relatively compact low-pressure mercury vapor discharge lamp.
Low-pressure mercury vapor discharge lamps are known in which the transverse connecting tube section is curved to a substantially U-shape (e.g., U.S. Pat. No. 4,353,007), or in which the transverse connecting tube section is bent once or several times (e.g., U.S. Pat. No. 4,481,442).
Low-pressure mercury vapor discharge lamps are also known in which the transverse connecting tube section is curved and constricted at portions adjacent to the longitudinal tube sections, or in which a straight transverse connecting tube section connects or bridges a pair of longitudinal tube sections at portions near their respective one ends.
A problem associated with these known low-pressure mercury vapor discharge lamps is a relatively low temperature region of the mercury vapor in the tubular discharge envelope, within the transverse connecting tube section.
For example, one conventional U-shaped type low-pressure mercury vapor discharge lamp (e.g., U.S. Pat. No. 4,353,007) has a curved transverse connecting tube section with a uniform diameter the same as that of the longitudinal tube sections. When this type of low-pressure mercury vapor discharge lamp is turned on, a low temperature region of the mercury vapor tends to occur in a region furthest from the discharge electrodes of the lamp. This occurs because the discharge of the mercury vapor is less activated in this region.
The low temperature region of the mercury vapor arises in the curved transverse connecting tube section. This causes the luminance of the low-pressure mercury vapor discharge lamp to be reduced at the low temperature region. Further, the mercury vapor pressure in the low-pressure mercury vapor discharge lamp lowers in accordance with the temperature of the mercury vapor in the low temperature region. As a result, the mercury vapor condenses and results in a so-called black shade, i.e., a film of mercury compound on the glass wall near the low temperature region.
Another conventional bent-type low-pressure mercury vapor discharge lamp (e.g., U.S. Pat. No. 4,481,442) has a flat transverse connecting tube section. The resulting discharge envelope has expanded its corner portions between the flat transverse connecting tube section and the longitudinal tube sections.
In this type of low-pressure mercury vapor discharge lamp, the discharge of the mercury vapor is less activated in regions near the corners of the expanded corner portions. The temperature of the mercury vapor is lowered at these regions in comparison to the temperature in other regions due to the reduced activation of the discharge. As a result, the luminance of the low-pressure mercury vapor discharge lamp is reduced at the low temperature regions. Morever, the mercury vapor pressure in the low-pressure mercury vapor discharge lamp lowers in accordance with the temperature of the mercury vapor in the low temperature region. The mercury vapor then condenses and results in a black shade film of mercury compound on the corner glass walls near these regions.
Referring to FIG. 1, the third conventional curved and constricted type low-pressure mercury vapor discharge lamp 10 has a transverse connecting tube section 11. In the manufacturing process of this type of low-pressure mercury vapor discharge lamp 10, a straight discharge envelope with a diameter DO is heated at its mid portion. Then, the straight discharge envelope is bent at the mid portion by using a mandrel. In the manufacturing process as described above, the discharge envelope tends to be reduced at portions 13a and 13b. On the other hand, the discharge envelope tends to be expanded at a middle portion 12.
Thus, the mid portion of the discharge envelope becomes a curved and constricted transverse connecting tube section 11, as shown in FIG. 1. The resulting curved and constricted transverse connecting tube section 11 has an elongated tube portion 12 with a diameter D1 at its mid length portion and a pair of reduced tube portions 13a and 13b, each having the diameter D2, in its end portions adjacent to longitudinal tube sections 14a and 14b, respectively. The diameters have the following relation: D2&lt;DO and D1&gt;DO.
In this type of low-pressure mercury vapor discharge lamp 10, the discharge of the mercury vapor is less activated in a region 18 near the elongated tube portion 12. This is because that the mercury vapor pressure lowers in the enlarged diameter portion 12 and the mercury vapor is less activated in a region 15 most far from discharge electrodes. The temperature of the mercury vapor lowers at the region 15 in comparison to the temperature in other regions due to the reduced activation of the discharge. This causes the luminance of the lamp 10 to be reduced at the low temperature region 15. Further, the mercury vapor in the low pressure mercury vapor discharge lamp 10 condenses and results in the so-called black shade, i.e., a film of mercury compound on the glass wall near the region 15, so that the luminance of the lamp 10 is further reduced at the low temperature region 18.
Referring to FIG. 2, the fourth bridged type low-pressure mercury vapor discharge lamp 20 has a straight transverse connecting tube section 21. The low-pressure mercury vapor discharge lamp 20 has a pair of longitudinal tube sections 24a and 24b, which are connected together at portions near adjacent ends via the straight transverse connecting tube section 21. Therefore, the longitudinal tube sections 24a and 24b have portions 26a and 26b which extend beyond the straight transverse connecting tube section 21.
In this type of low pressure mercury vapor discharge lamp 20, the discharge of the mercury vapor is less activated in regions 25a and 25b in the extended portions 26a and 26b. The temperature of the mercury vapor lowers at the regions 25a and 25b in comparison to the temperature in other regions due to the reduced activation of the discharge. This causes the luminance of the lamp 20 to be reduced at the low temperature regions 25a and 25b. Further, the mercury vapor in the low-pressure mercury vapor discharge lamp 20 condenses and results in the so-called black shade, i.e., a film of mercury compound on the glass wall near the regions 25a and 25b, so that the luminance of the lamp 20 is further reduced at the low temperature regions 25a and 25b.